diff --git a/examples/rack/README.md b/examples/rack/README.md
index 193ab20c..aecdfc6c 100644
--- a/examples/rack/README.md
+++ b/examples/rack/README.md
@@ -23,7 +23,7 @@ output of `/metrics` and terminate.
 Start a Prometheus server with the provided config:
 
 ```bash
-prometheus -config.file ./prometheus.yaml
+prometheus -config.file ./prometheus.yml
 ```
 
 In another terminal, start the application server: